performance/io-threads: Fix range-check for least-rate-limit

The issue could be fixed with .validate=GF_OPT_VALIDATE_MIN. But
adding max value is more robust.

Change-Id: Ia69c6f86855dbd34a26e20391e77bfa0f796a200
BUG: 923573
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/4698
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>